An aircraft airspeed unit measures the difference of pressure between the Pitot ( or total pressure) and the Static pressure.
The Pitot pressure is obtained from the Pitot Tube and is a measure of ram air pressure (the air pressure created by vehicle motion or the air ramming into the tube). The Pitot tube is most often located on the wing or front section of an aircraft, facing forward, where its opening is exposed to the relative wind. When airspeed increases, the ram air pressure is increased, which can be translated by the airspeed indicator.
The static pressure is obtained through a static port which most often is a flush-mounted hole on the fuselage of an aircraft located where it can access the air flow in a relatively undisturbed area. Some aircraft may have a single static port, while others may have more than one. A Pitot-static tube effectively integrates the static ports into the Pitot probe. It incorporates a second coaxial tube (or tubes) with pressure sampling holes on the sides of the probe, outside the direct airflow, to measure the static pressure. When the aircraft climbs, static pressure will decrease.
This unit contains a simple aneroid capsule which is coupled to an arm sliding along a wire wound resistor. This simple arrangement transmits the difference signal between the Pitot and Static pressure to a remote indicator for Indicated Air Speed. The signal is also fed to a locking solenoid used in Height Lock mode to maintain barometric height. On the Buccaneer for example this was important in its low level mode and the Height Lock input forms part of the Autopilot and controls the tailplane angle.
Marconi Collection Ref: 2024

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